This evening we watched the 1954 Halas/Batchelor animated version of Animal Farm, which was pretty good. It's only about 70 minutes long, and I'd seen at least bits of it long ago when they used to have school programmes on the BBC in the daytime - I have a memory, possibly imagined, of seeing a bit of it at my granny's house - possibly interspersed with documentary bits explaining the allegory, though I'm not sure if I knew it was an allegory the first time I saw it. Anyway, it is a beautifully constructed allegory. (Maisie's doing it in English at the moment.)
I was surprised at the ending, which is different from the famous ending of the book - which I checked in the copy on the shelf behind me (my Prebble, Orwell, Milligan shelf). It depicts a further uprising against the pigs, and we wondered if this was a politically-motivated change. Near the start of the Wikipedia entry for the film it says, 'The C.I.A. paid for the filming, part of the U.S. cultural offensive during the Cold War, and influenced how Orwell's ideas were to be presented'. I don't know how true this is.